The present invention relates to resonant circuits and, more particularly, deactivateable resonant security tags for use with electronic security and other systems for the detection of unauthorized removal of articles.
Electronic article surveillance (EAS) systems for detecting and preventing theft or unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, are well known and widely used. In general, such security systems employ a label or security tag which is affixed to, associated with, or otherwise secured to an article or item to be protected or its packaging. Security tags may take on many different sizes, shapes, and forms, depending on the particular type of security system in use, the type and size of the article, etc. In general, such security systems detect the presence of an active security tag as the security tag (and thus the protected article) passes through a surveillance zone or passes by or near a security checkpoint.
Certain prior art security tags work primarily with radio frequency (RF) electromagnetic field disturbance sensing electronic security systems, such as, but not limited to those disclosed in U.S. Pat. No. 3,810,147 entitled "Electronic Security System", U.S. Pat. No. 3,863,244 entitled "Electronic Security System Having Improved Noise Discrimination", and U.S. Pat. No. 5,276,431 entitled "Security Tag For Use With Article Having Inherent Capacitance", and their commercially available implementations and counterparts. Such electronic security systems generally establish an electromagnetic field in a controlled area through which articles must pass when being removed from the controlled premises. A tag having a resonant circuit is attached to each article, and the presence of the resonant circuit in the controlled area is sensed by a receiving system to denote the unauthorized removal of an article. The resonant circuit can be deactivated, detuned, shielded, or removed by authorized personnel from any article authorized (i.e. purchased or checked out) to be removed from the premises, thereby permitting passage of the article through the controlled area without alarm activation.
Security tags can be affixed to or associated with the article being secured or protected in variety of manners. Removal of a tag which is affixed to an article can be difficult and time consuming and, in some cases, requires additional removal equipment and/or specialized training. Detuning the security tag, for instance, by covering it with a special shielding device such as a metallized sticker, is also time consuming and inefficient. Furthermore, both of these deactivation methods require the security tag to be identifiable and accessible, which prohibits the use of tags embedded within merchandise at undisclosed locations or tags concealed in or upon the packaging.
The trend in the electronic article surveillance industry now is to install the tag in a product at the time the product is being manufactured, since at this stage, it is relatively inexpensive to install the tag and because the tag may be concealed or hidden from view. Embedding the tag in the product or the product packaging requires that the tag be remotely deactivateable.
Electronic deactivation involves altering or changing the frequency at which the tag circuit resonates, or preventing the tag circuit from resonating altogether, so that the tag is no longer detected as it passes through the surveillance zone. Such tags can be conveniently deactivated at a checkout counter or other such location by being momentarily placed above or near a deactivation device which subjects the tag to electromagnetic energy at a power level sufficient to cause one or more components of the security tag's resonant circuit to either short circuit or open, depending upon the detailed structure of the tag.
There are many methods available for achieving electronic deactivation. One method of deactivation involves shorting the tag's resonant circuit. This type of electronically deactivateable tags include a weak link created by forming a dimple in the tag which brings more closely together plates of a capacitor formed by the metallizations of two different parts of the tag's resonant circuit on opposite sides of the tag substrate, thereby allowing electrical breakdown at moderate power levels. Such a breakdown causes a short circuit between the two metallizations.
Another deactivation method is disclosed in U.S. Pat. No. 4,021,705 to Lichtblau, which discloses a tag resonant circuit having a fusible link which bridges one or more turns of a planar inductor. Referring to FIG. 1, a conductive path 10 which forms a part of a turn of an inductor of a resonant circuit includes a fusible link 12. The fusible link 12 comprises a narrowed or necked-down portion of the conductive path 10. The fusible link 12 is burned out by the application of energy higher than that employed for detection to either activate or deactivate the tuned circuit. That is, the fusible link 12 is dimensioned to fuse upon flow of a predetermined high current therethrough caused by an applied electromagnetic field, which short circuits the inductor. Shorting the inductor lowers the Q of the resonant circuit, which increases its resonant frequency. Although effective, this method requires relatively high current to break the fuse. In addition, it is often difficult to consistently and repeatedly form such a fuse using standard macro etching techniques generally used to fabricate the tags.
Yet another deactivation method is disclosed in U.S. Pat. No. 4,835,524 to Lamond et al. Referring to FIG. 2, a conductive path 14 includes a gap or break which is bridged by a fuse 16. The fuse 16 comprises a conductive material, such as a conductive ink mixed with an accelerator substance, such as potassium permanganate, which acts as an explosive-type agent to mechanically assist the opening of the fuse. This is known as an explosive type of fuse. The inclusion of the accelerator substance makes the fuse 16 very sensitive to induced current.
There is a need for a tag having a deactivateable resonant circuit which is effective, can be deactivated using moderate power, and may be manufactured at a very low cost.